Rock Grove Township is located in Stephenson County, Illinois. As of the 2010 census, its population was 1,388 and it contained 761 housing units. The unincorporated community of Rock Grove and part of Lake Summerset are located in the township, and once Duncannon was located here also.
Rock Grove is Township 28 (part) and 29 North, Range 8 (part) and 9 East of the Fourth Principal Meridian.
According to the 2010 census, the township has a total area of , of which (or 99.51%) is land and (or 0.49%) is water.